Scrutiny of a key profit source for the drug industry is increasing. That means it is getting harder for investors to forecast industry growth.

The Department of Health and Human Services’ Office of Inspector General last week revoked a key favorable advisory opinion for Caring Voice Coalition, one of the largest U.S. charities that helps patients afford insurance copayments for their medicine. The OIG said in a letter, first reported by Bloomberg News, that the charity had provided undue influence to its donors over how the charity was run. The charity said that the loss of the OIG stamp of approval would likely cause it to cease operating.

Copay-assistance charities are a common way for pharma companies to help patients cope with high drug prices. The eight largest drug companies donated more than $1 billion in 2014 to charities that help Medicare patients afford drugs, according to Bernstein Research. Since copays are typically a small fraction of a drug’s total cost, those donations are also a great investment. Analysts at Citi Research calculated earlier this year that $1 million in charitable donations could generate as much as $21 million in sales.

But certain kinds of patient assistance can violate anti-kickback laws for patients that have government-sponsored health insurance like Medicare. And scrutiny of the practice is mounting—the U.S. attorney’s office in Massachusetts has subpoenaed numerous drug companies and pharmacy-benefit managers for documents relating to their patient-assistance programs.

That doesn’t necessarily mean every drug company that has received a subpoena is in legal trouble. Any crackdown on charities could result in disappointing future sales numbers for drugs that investors care about, though.

Those disappointments can come without much warning. Sales of high-price prostate cancer drugs owned by
Pfizer Inc.
and
Johnson & Johnson
fell sharply earlier this year after a slowdown in charitable contributions led to more giveaways of free drugs. Sales of the drugs eventually rebounded, and the diverse product offerings of both companies, as well as a roaring bull market, insulated them from meaningful share-price pressure.

But the fact that nearly all pharma companies donate to patient-assistance charities, coupled with the opaque nature of government investigations, make predicting the next sales shortfall very difficult. Worse, most drug companies derive a high share of revenues from relatively few products.

With the sector’s stock market glow already fading, investors probably won’t be as forgiving next time.